Hioki DM7275-01 - Voltage Measurement; X84; Thermal Electromotive Force

Appx.5
Causes of Error in Voltage Measurement
Appx.3 Causes of Error in Voltage Measurement
Thermal electromotive force
Thermal electromotive force is the potential difference that occurs at connections between different
metals, for example between the measurement cable’s metal pins and the measurement target.
When this thermal electromotive force is large, an error will be introduced to measurements (see
gure below).
Temperature t
1
Temperature t
2
Metal A
Metal B
Instrument
Instrument
Figure. Occurrence of Thermal Electromotive Force
The magnitude of thermal electromotive force varies with the combination of metals involved. In
general, the larger the temperature difference, the larger the thermal electromotive force.
Since the instrument’s measurement terminals are made of copper, it is possible to minimize the
effects of thermal electromotive force by using copper for contacts such as banana terminals
and crimp terminals and as a wiring material. Typical banana terminals and crimp terminals use
brass as a material, making them poorly suited for use when making precise, microvolt-scale
measurements. Use cables with low electromotive force with copper terminals as measurement
cables when calibrating the instrument.
Examplesofhighthermalelectromotiveforce
• Setups in which the measurement circuit contains a fuse, temperature fuse, thermistor, bimetal
components, or thermostat
• Setups in which single stable relay contacts are used to switch measurement circuits
• Setups in which the instrument is connected to the measurement target by means of alligator
clips
• Setups in which the measurement terminals or measurement cable metal pins are held by hand
• Setups in which the temperature of the measurement target or instrument is unstable
• Setups in which different wiring materials are used for the HIGH and LOW terminals
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Appx.
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